A slightly more dramatic demonstration of buoyancy than simply having a series of balls float in water is to float soap bubbles on a layer of carbon dioxide.

**Equipment**

- Fish tank
- Large bag of baking soda
- Gallon jug of vinegar
- Soap bubble blower

**Demonstration**

- Pour the baking soda into the fish tank and then pour in the vinegar. It will bubble vigorously creating the CO
_{2} layer. You need quite a lot of each.
- Wait for all the bubbling to stop. You may need to mix it up a bit, but not too much so that you do not mix up the CO
_{2} layer too much.
- Blow the bubbles into the fish tank. If it all worked out then the soap bubbles should appear to float in mid-air.

**Analysis**

This is a largely qualitative demonstration, though you can make up some numbers to do a simple calculation. Take a 1 cm radius soap bubble floating in the CO_{2} layer.

If you assume that the bottom half of the bubble is in the CO_{2} layer and the top half is in the air then you can calculate the buoyancy force acting on the bubble due to the CO_{2} and the air

F_{B}= (4/3)πr^{3} ((½)ρ_{CO2} g + (½)ρ_{air} g)

This is balanced by the weight of the air in the bubble plus the weight of the soap.

W= (4/3)πr^{3} ρ_{air} g + 4πr^{2}Tρ_{soap}g

where T is the thickness of the soap film. The balance then becomes

F_{B}= W or (4/3)πr^{3} (½ρ_{CO2} g + ½ρ_{air} g)= (4/3)πr^{3} ρ_{air} g+ 4πr^{2}Tρ_{soap}g or (½)r (ρ_{CO2} + ρ_{air} )=r ρ_{air} + 3Tρ_{soap}

which leads to

T=(½)r( ρ_{CO2} – ρ_{air})/3ρ_{soap}

Substituting material properties (ρ_{CO2}=1.98 kg/m^{3} ρ_{air}=1.23 kg/m^{3} and ρ_{soap}=900 kg/m^{3} ) into the equation give T=1.4 μm. This is consistent (at least in order of magnitude) with thin-film Interference estimates.

An index of all the demonstrations posted on this blog can be found

here. Don’t forget to follow

@nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (

nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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